1. Field of the Invention
The present invention relates generally to a computer system including a battery-driven personal computer, and more particularly to a control means, incorporated in the computer system, for controlling the power supply to a storage unit, so as to save the power provided by the battery.
2. Description of the Related Art
In recent years, lap top type personal computers integrally comprising a keyboard and a display unit have come into wide use. Some of the personal computers of this type are provided with not only a floppy disk drive (FDD) but also a hard disk drive (HDD) as standard equipment. Further, some of the personal computers of that type can be driven not only on a commercially-available AC power source but also on a chargeable battery.
In a personal computer which operates on a battery, the saving of power is one of the important factors in determining the performance of the personal computer. To provide an auto power off mode for the computer is one way for saving power as much as possible. The auto power off mode is a mode wherein the power supply to a given unit (e.g. a display unit) is automatically stopped if the keyboard is not operated for a predetermined length of time. The auto power off mode is described in U.S. patent application Ser. No. 378,755, filed Jul. 12, 1989, for example.
Some of the personal computers of the type mentioned above are provided with a resume mode, in which the data is transferred from registers even after into a backup memory, thereby securing the data when the power switch is turned off. Where a computer is set in this resume mode, the operating system (OS) need not be loaded when the computer is turned on, and a program can be restarted in the same state as it was held immediately before the computer was turned off. Let it be assumed that the power provided by the battery decreases and the battery is replaced with another or is charged. In this case, the computer, if set in the resume mode, need not be initialized when it is switched on, and data processing can be resumed. The resume mode is described in U.S. patent applications Ser. Nos. 343,356, 507,490, and 540,643, for example.
By utilization of both the auto power off mode and the resume mode, the battery-driven personal computer operates in a reliable manner without deteriorating in performance. However, if the battery-driven personal computer is provided with a storage unit which consumes a large amount of power, such as an HDD, the load on the battery increases. In spite of this load increase, it is impossible, as a matter of fact, to increase the capacity of the battery incorporated in a lap top type personal computer. Therefore, the time for which the battery can be used is limited where the computer incorporates a storage unit that consumes a large amount of power. As a result, the performance of the computer is degraded.